In quantum physics, the divisions between object and observer—the systems and environment—become blurred. Because any measuring device is governed by the laws of quantum mechanics, the act of measurement involves an interaction between two quantum systems. The exact mechanisms by which this works are still unclear in many instances, but much of the quasi-mystical language once used to describe quantum mechanics has given way to precise scientific descriptions.

One remaining frontier is comprehension of how systems gradually lose coherence via interactions with their environment, which prevents their usefulness in quantum computing. A new set of experiments by Yinnon Glickman, Shlomi Kotler, Nitzan Akerman, and Roee Ozeri revealed part of the mechanism by which environment disrupts quantum systems: photons. They found that photons that interacted with a quantum system can end up correlated with the system's state, the hallmark of entanglement. By careful preparation of the atom's state, it may be possible to reduce the loss of quantum information to the environment, and thus extend the life of these systems.

Measurement in quantum physics transforms an indeterminate system—one that behaves as if it's in a number of different states simultaneously—into one with a definite set of physical attributes. For example, the spin of an electron cannot typically be known in the absence of a measurement. However, sending the atom through a (nonuniform) magnetic field will deflect it either up or down relative to the field, showing that the electron is either aligned with or aligned against the magnet.

Since that outcome will happen whichever way the magnetic field is oriented, the electron's state is indeterminate before the measurement, but "collapses" into one of two alternatives after the fact. Those two alternatives comprise the "measurement basis," relative to the magnetic field; a different orientation will yield a different measurement basis.

Even if a quantum system is in a definite state (say after a measurement), interactions with its environment will tend to nudge it back toward indeterminacy. This process is known as decoherence, and it stands as one of the biggest obstacles to quantum computing. In essence, the environment itself—which includes ambient photons that make up the electromagnetic field—is performing "measurements" on the system. If these (or any other) measurements involve light, then the interaction creates entanglement between the quantum states of the photons and the matter.

In the current experiment, the researchers trapped a single strontium (Sr) atom in crossed electromagnetic fields. Strontium has an unpaired electron in its outermost shell, which dictates the spin of the atom. The experimenters subjected this spin to an additional weak magnetic field, coaxing it into a preferred quantum state. They then shone a laser on the atom in a direction perpendicular to the magnetic field, and collected the photons that scattered.

The scattering process involves an atom absorbing a photon, which induces a transition between two quantum states inside the atom. Subsequently, the atom undergoes a second transition, emitting a new photon. So, one photon entered, one photon left, but the second takes some information with it: the polarization of the photon depends on the spin state of the electron in the strontium atom. By measuring the polarization of these scattered photons, the researchers could reconstruct the quantum state transitions of the atom.

To reconstruct the full physical system, the researchers prepared the atoms so that the electron spin started in one of several different directions, then compared the photon polarizations after scattering. The outcomes were very different: if the electron spin was aligned with or against the direction of photon scatter, then the light came out with one of two simple polarization orientations, corresponding to the measurement basis. However, for any other electron spin direction, the photons came out entangled with the state of the atom.

That meant that the result of the polarization measurement was correlated with the state of the atom. The outcome was a mixture of possible polarizations and, in most cases, they results didn't correspond to one or the other state in the measurement basis.

The finding is significant because the quantum state of a trapped atom has been used as a qubit in a quantum computer. Unfortunately, according to quantum physics, the environment contains ambient photons, which interact with any atom, and this can lead to decoherence. In effect, the environment is erasing the state needed for computations.

This experiment showed that orienting the detection devices in a clever way could avoid this form of decoherence, by exploiting scattering directions in which entanglement between environment photons and the atom's spin state does not take place. While this may sound simple, in practice it would be somewhat more challenging. Nevertheless, these new results indicate how future experiments could compensate for some aspects of decoherence.

Somehow I'm not understanding the newness of this. It sounds like pretty much another way of saying one of the most obvious things in quantum mechanics: an atom interacting with a photon can produce a particular form of entanglement, or not, depending on the measurement basis; information preserved by the outgoing photon causes entanglement, and information that isn't preserved isn't measured and is therefore unentangled.

What am I missing? Also, I don't see how this is particularly useful for quantum computers, because using this to avoid decoherence requires a particular scattering alignment between the atom and the photon, and that is exactly what you can't control with an environmental photon: it is noise, and therefore randomly timed and oriented.

You can say that again. Don't know what it is. It's a mess from where I look at it.

justthinkit wrote:

Yinnon Glickman, Shlomi Kotler, Nitzan Akerman, and Roee Ozeri

That's so weird. My children are named Yinnon, Shlomi, Nitzan, and Roee.

Please stop for a moment. You don't want to say this here. I happen to know, Islamic culture for an example. Islam often named their names after their great-great grandparents, great grandparents, grandparents, fathers, uncles. Their names mostly ended up with 10 firstnames or longer just as the example you have given here. Question: What this make you your relationship with those guys? <sorry I couldn't resist> :-)

What am I missing? Also, I don't see how this is particularly useful for quantum computers, because using this to avoid decoherence requires a particular scattering alignment between the atom and the photon, and that is exactly what you can't control with an environmental photon: it is noise, and therefore randomly timed and oriented.

Well, the article emphasizes "measurement" and "coherence". So I guess it's about reliable measurement and states. I can imaging that enhancing coherence can lessen power requirement to "reformat" qbits e.g. memory refresh.

Somehow I'm not understanding the newness of this. It sounds like pretty much another way of saying one of the most obvious things in quantum mechanics: an atom interacting with a photon can produce a particular form of entanglement, or not, depending on the measurement basis; information preserved by the outgoing photon causes entanglement, and information that isn't preserved isn't measured and is therefore unentangled.

What am I missing?

The fact that entanglement is very common. Once, it was thought that it was very, very rare but it has become obvious that it is happening all the time.

Quote:

Measurement in quantum physics transforms an indeterminate system—one that behaves as if it's in a number of different states simultaneously—into one with a definite set of physical attributes.

This sounds like the collapse of superposition to me. Entanglement and superposition are the two sides of the same coin; they are closely related.

Somehow I'm not understanding the newness of this. It sounds like pretty much another way of saying one of the most obvious things in quantum mechanics: an atom interacting with a photon can produce a particular form of entanglement, or not, depending on the measurement basis; information preserved by the outgoing photon causes entanglement, and information that isn't preserved isn't measured and is therefore unentangled.

What am I missing?

The fact that entanglement is very common. Once, it was thought that it was very, very rare but it has become obvious that it is happening all the time.

Quote:

Measurement in quantum physics transforms an indeterminate system—one that behaves as if it's in a number of different states simultaneously—into one with a definite set of physical attributes.

This sounds like the collapse of superposition to me. Entanglement and superposition are the two sides of the same coin; they are closely related.

No, entanglement has always been known to be common, and this doesn't change that. But normally, the entanglement is very short (locally). It is LONG entanglements in a localized area that are rare.

Sorry if this sounds like a lecture, but it is really interesting and rarely described well:

In every single particle interaction that produces multiple particles that has ever taken place since the big bang, the outgoing particles are entangled to some degree, *relative* to the rest of the universe that they haven't yet interacted with. They are entangled to the degree that you could reconstruct information about the state of one of the particles from the state of the other (for example, that their trajectories must intersect if you extrapolate back in time). Entanglement is relative, and it exists for exactly that part of the universe that hasn't yet received any information in any form that might in theory with infinite resources in time and space be used to reconstruct information that would collapse the original superposed value.

Everything that hasn't yet interacted with the system sees a superposition, and an entanglement, and everything that has interacted with the system (measured i.e. encoded invertibly a superposed property) enters the entanglement, and doesn't see the entanglement anymore: the superposition has become classical in that context. But *it* is now entangled too, relative to the complement of the transitive closure of the interaction (the rest of the universe).

So entanglement appears to be rare BECAUSE WE HAVE BECOME PART OF THE ENTANGLEMENT and so don't see it anymore. But to the rest of the universe, including every single thing farther away than light could have traveled since the event happened, the entanglement still exists, because the information has not been received i.e. measured yet there, so the superposition has not yet collapsed from that viewpoint.

And as a side note there are plenty of long lived entanglements around, it's just that we aren't aware of them for the very reason they exist: we haven't interacted with them. Every non-virtual interaction in space has outgoing particles which diverge, and it is a lottery how long it takes for them to encounter their next interaction. It might be billions of years, which is how our telescopes pick up photons that were emitted 13 billion years ago. Relative to the particle interaction where that photon originated, we pop a superposition bubble that lasted 13 billion years, from our point of view: we have finally measured that photon, a tiny superposition collapses and we enter the entanglement, and it is gone, to us. Note that these are long lived, but not local, so they aren't counterexamples to my rarity claim.

One way to think about why long-lived entanglement is locally rare is to note that entanglement exists where (theoretical) ignorance of a superposed value must exist, and ignorance is harder to maintain for very long the closer you are to where the value is.

One way to think about why long-lived entanglement is locally rare is to note that entanglement exists where (theoretical) ignorance of a superposed value must exist, and ignorance is harder to maintain for very long the closer you are to where the value is.

If you use a word like ignorance, you'll find that a lot of people will insist that only humans can made measurements and are therefore special.

And the way to preserve entanglements a long time locally is simply not to measure them; a task that is easier to talk about than to do.

In every single particle interaction that produces multiple particles that has ever taken place since the big bang, the outgoing particles are entangled to some degree, *relative* to the rest of the universe that they haven't yet interacted with. They are entangled to the degree that you could reconstruct information about the state of one of the particles from the state of the other (for example, that their trajectories must intersect if you extrapolate back in time). Entanglement is relative, and it exists for exactly that part of the universe that hasn't yet received any information in any form that might in theory with infinite resources in time and space be used to reconstruct information that would collapse the original superposed value.

Everything that hasn't yet interacted with the system sees a superposition, and an entanglement, and everything that has interacted with the system (measured i.e. encoded invertibly a superposed property) enters the entanglement, and doesn't see the entanglement anymore: the superposition has become classical in that context. But *it* is now entangled too, relative to the complement of the transitive closure of the interaction (the rest of the universe).

You have a particle that interacted with the system and measured spin up for the system.For the rest of the universe, the spin of the system remains up and down, in superposition.

Now - another particle interacts with the system.

1.Will it always measure spin up for the system? If so, the system is in an illusion of superposition (regarding spin) for the rest of the universe (the system has a definite value for spin; it's just that the rest of the universe doesn't have enough information to figure out which value this is). It is an illusion of superposition because true superposition means the system has both values for spin at the same time (as Bell's inequalities prove).

2.Can it measure spin down for the system, too? If so, the system is collapsed to spin up for someone AND collapsed to spin down for someone else (attention - this is NOT superposition where both values for spin coexist in the system FOR ALL OBSERVERS). Indeed, the 'collapsed to spin up for someone AND collapsed to spin down for someone else' is not a state systems can be in/are found in (even quantum mechanical systems). That's why von Neumann called this the measurement problem.

In every single particle interaction that produces multiple particles that has ever taken place since the big bang, the outgoing particles are entangled to some degree, *relative* to the rest of the universe that they haven't yet interacted with. They are entangled to the degree that you could reconstruct information about the state of one of the particles from the state of the other (for example, that their trajectories must intersect if you extrapolate back in time). Entanglement is relative, and it exists for exactly that part of the universe that hasn't yet received any information in any form that might in theory with infinite resources in time and space be used to reconstruct information that would collapse the original superposed value.

Everything that hasn't yet interacted with the system sees a superposition, and an entanglement, and everything that has interacted with the system (measured i.e. encoded invertibly a superposed property) enters the entanglement, and doesn't see the entanglement anymore: the superposition has become classical in that context. But *it* is now entangled too, relative to the complement of the transitive closure of the interaction (the rest of the universe).

You have a particle that interacted with the system and measured spin up for the system.For the rest of the universe, the spin of the system remains up and down, in superposition.

Now - another particle interacts with the system.

1.Will it always measure spin up for the system? If so, the system is in an illusion of superposition (regarding spin) for the rest of the universe (the system has a definite value for spin; it's just that the rest of the universe doesn't have enough information to figure out which value this is). It is an illusion of superposition because true superposition means the system has both values for spin at the same time (as Bell's inequalities prove).

2.Can it measure spin down for the system, too? If so, the system is collapsed to spin up for someone AND collapsed to spin down for someone else (attention - this is NOT superposition where both values for spin coexist in the system FOR ALL OBSERVERS). Indeed, the 'collapsed to spin up for someone AND collapsed to spin down for someone else' is not a state systems can be in/are found in (even quantum mechanical systems). That's why von Neumann called this the measurement problem.

These are only problems in some interpretations of quantum mechanics. One of the many advantages of the Many Worlds Interpretation is that these problems trivially go away. The superposition remains real, but observers who see different collapsed/decohered measurement values are simply in different branches of the universe and can't communicate anymore.

It IS real superposition, because MWI doesn't just allow branching- merging occurs too. In other words, you can have two alternate future histories that separate, and then rejoin. But the requirement for rejoining is that no information recording the disparate measurements can survive. But more indirect information CAN survive that indicates that indeed the different branches existed.

This isn't hand waving, there is experimental evidence that shows how this process works: delayed choice quantum erasure. You can send a photon through a double slit, and put measuring devices in the slits which measure which slit the photon went through. If you do the measurement and record the result in a persistent fashion, it acts like a particle and produces no interference pattern. So after the measurement, the superposition is collapsed, and the other possibility is GONE. Yet, it has been discovered if you do the measurement, collapse the superposition, but AFTERWARDS merge the information from the two detectors in a way that destroys the record of which slit it went through, the interference pattern REAPPEARS.

This looks like retrocausality, but it is really just the process we have been talking about: we know that the measurement was made, so we know that collapse happened, although we don't know which value it saw. And yet we still see an interference pattern, which means that the photon passed through both slits, despite the fact that because of our measurement it had to have passed through only one. The only way to make sense of this is to understand that the interference pattern we see at the end is the merging of the two measurement universes back into a superposition. Both values for the measurement had to have been made for the interference pattern to exist, but the conditions for that to occur are that no possible recording of which-way information survives.

In every single particle interaction that produces multiple particles that has ever taken place since the big bang, the outgoing particles are entangled to some degree, *relative* to the rest of the universe that they haven't yet interacted with. They are entangled to the degree that you could reconstruct information about the state of one of the particles from the state of the other (for example, that their trajectories must intersect if you extrapolate back in time). Entanglement is relative, and it exists for exactly that part of the universe that hasn't yet received any information in any form that might in theory with infinite resources in time and space be used to reconstruct information that would collapse the original superposed value.

Everything that hasn't yet interacted with the system sees a superposition, and an entanglement, and everything that has interacted with the system (measured i.e. encoded invertibly a superposed property) enters the entanglement, and doesn't see the entanglement anymore: the superposition has become classical in that context. But *it* is now entangled too, relative to the complement of the transitive closure of the interaction (the rest of the universe).

You have a particle that interacted with the system and measured spin up for the system.For the rest of the universe, the spin of the system remains up and down, in superposition.

Now - another particle interacts with the system.

1.Will it always measure spin up for the system? If so, the system is in an illusion of superposition (regarding spin) for the rest of the universe (the system has a definite value for spin; it's just that the rest of the universe doesn't have enough information to figure out which value this is). It is an illusion of superposition because true superposition means the system has both values for spin at the same time (as Bell's inequalities prove).

2.Can it measure spin down for the system, too? If so, the system is collapsed to spin up for someone AND collapsed to spin down for someone else (attention - this is NOT superposition where both values for spin coexist in the system FOR ALL OBSERVERS). Indeed, the 'collapsed to spin up for someone AND collapsed to spin down for someone else' is not a state systems can be in/are found in (even quantum mechanical systems). That's why von Neumann called this the measurement problem.

These are only problems in some interpretations of quantum mechanics. One of the many advantages of the Many Worlds Interpretation is that these problems trivially go away. The superposition remains real, but observers who see different collapsed/decohered measurement values are simply in different branches of the universe and can't communicate anymore.

It IS real superposition, because MWI doesn't just allow branching- merging occurs too. In other words, you can have two alternate future histories that separate, and then rejoin. But the requirement for rejoining is that no information recording the disparate measurements can survive. But more indirect information CAN survive that indicates that indeed the different branches existed.

The Many Worlds Interpretation may solve the measurement problem, but it replaces this with a lot of other problems. For starters, it breaks all conservation laws known to mankind, the laws of thermodynamics, etc. And if there's the slightest non-linearity between the worlds, this breaking is not only philosophical, but practical as well.

The Many Worlds Interpretation may solve the measurement problem, but it replaces this with a lot of other problems. For starters, it breaks all conservation laws known to mankind, the laws of thermodynamics, etc. And if there's the slightest non-linearity between the worlds, this breaking is not only philosophical, but practical as well.

No, it doesn't break the conservation laws. The current "mainstream" physics consensus is that the MWI is at the very least one of several valid interpretations, and that different interpretations are pretty much mathematically equivalent (mathematically although not intuitively). Certainly problems with obvious things like the laws of thermodynamics would have been dealt with by now.